It has become a widely accepted practice in the textile industry to treat textile materials, especially cellulosic materials such as cotton or rayon, but also wool, silk and various synthetic fibers, for the purpose of rendering the material resistant to the action of flame and heat. Such practice has assumed increasing importance with the adoption of legislation designed to protect the public against the hazards of flammable fabrics in articles of clothing, toys, household articles such as curtains and drapes, and the like.
A broad group of flameproofing agents or flame retardants which have received extensive attention is represented by the inorganic and the organic phosphorus compounds. One theory to explain why such phosphorus compounds function as flame retardants for substrates, especially organic substrates such as cellulose, is that they produce phosphorus pentoxide during exposure to flame. The liberated P.sub.2 O.sub.5, which is a Lewis acid, thereupon acts on the organic fiber substrate to dehydrate it, forming water and carbon, which are less flammable than the gaseous and tarry products of ordinary degradation. Another theory is that the phosphorus compounds change the path of degradation to prevent the formation of levoglucosan, to increase the amount of carbon, water and carbon dioxide and to reduce the ammount of flammable, volatile gases and flammable tars. Examples of agents which have been employed in the prior art for this purpose include salts of orthophosphoric acid and other acids of phosphorus, such as diammonium phosphate, and salts formed from mixtures of an aliphatic organic base and an acid of phosphorus. The acids of phosphorus which have been used to form salts with organic bases include, for example, orthophosphoric acid, phosphoric acid, pyrophosphoric acid, and methyl phosphonic acid. Typical organic bases include cyanamide and urea. These salts may be employed per se, or in association with haloalkyl phosphonic acid derivatives and phosphate esters, which are also flameproofing agents.
Another type of phosphorus-containing flameproofing agent comprises the haloalkylphosphonic acids and their salts, such as chloromethylphosphonic acid. These compounds react with a portion of the hydroxyl groups of the cellulose or other hydroxyl-containing fiber molecule, thereby forming ethers and chemically modifying the textile material by incorporating phosphorus into the fibers.
One of the drawbacks of such phosphorus-containing flame retardants has been their ability to undergo ion exchange when the textile material is laundered in water containing alkali metal or alkaline earth metal compounds which causes a loss of fire retardance. Thus, textiles can be rendered fire retardant by the application of a compound such as diammonium phosphate to form a monophosphate ester of cellulose, Cell-O-P(O)-(OH).sub.2, or an ammonium salt thereof. However, this fire retardance can be lost in a single laundering in water containing even small amounts of alkali metal or alkaline earth metal compounds, because of the conversion of this readily decomposable monophosphate ester or ammonium salt into an alkali metal or alkaline earth metal salt which does not decompose readily into P.sub.2 O.sub.5 when heated.
These ion exchange properties are exhibited by a number of phosphorus flame retardants. The phosphorus can be in a compound linked chemically to the cellulose or in a compound which is deposited as an insoluble deposit in or on the textile fibers. The groups which are usually associated in ion exchange properties are acidic OH groups. But, ion exchange can take place with other groups which are capable of linking with metal ions such as alkali metal and alkaline earth metal ions. The groups can be attached to the phosphorus atoms or attached to other atoms which are on the textile.
Normally the metal ions that cause the most trouble during laundering are the so-called hard water ions such as calcium and magnesium. We will refer to hard water ions as being the ones causing loss of fire retardance, but it is to be understood that both the usual hard water ions and ions such as sodium and potassium can be ion exchanged by the fire retardant fabric with subsequent loss of fire retardance. The process of this invention protects the fabric not only from hard water ions, but also all of the alkali metal and alkaline earth metal ions.
No matter what the mechanism of the ion exchange, the effect on the fire retardance of the textile laundered in hard water is the same: a loss of fire retardance associated with a pick-up of hard water ions. This is theoretically due to these metals, such as calcium, tying up phosphorus during the combustion process so that phosphorus may not function as a flame retardant.
Thus, for example, it is known that cellulose can be phosphonomethylated by treatment with the sodium salt of chloromethylphosphonic acid in accordance with the equation: EQU ClCH.sub.2 P(O)(ONa).sub.2 + Cell-OH + NaOH .fwdarw. Cell-OCH.sub.2 P(O)(ONa).sub.2 + NaCl.
When the treated textile is acidified with an acid such as hydrochloric acid, the sodium salt is transformed into the acid form of phosphonomethylated cellulose which has two free acid groups on the phosphorus atom. Whether it is in the salt form or in the free acid form, it is capable of picking up calcium ions by ion exchange when it is laundered in hard water, to form a calcium phosphonate salt which does not readily decompose.
A similar tendency toward ion exchange with calcium and other ions in hard water is found in textiles treated with salts or organic bases and phosphoric acid, which present some acid groups for ion exchange.
It has been proposed (see U.S. Pat. No. 2,728,680) to apply a mixed solution of a soluble chloride of tetravalent titanium and a soluble inorganic phosphate to cellulosic material as a flame retardant. The mixture is gelatinized and adhered onto the fabric. The solutions, however, only contain a small amount of phosphorus (0.02 to 0.17 part of phosphorus per part of titanium) and attempts to add higher proportions of phosphorus to such titanium tetrachloride solutions result in precipitation of titanium phosphate. The addition to the solution of antimony trichloride, another known flame retardant agent, is thus recommended in the patent to improve the fire resistant properties of the treated fabric, particularly after laundering.
In U.S. patent application Ser. No. 307,796, filed Nov. 20, 1972, now U.S. Pat. No. 3,827,907, there is disclosed a novel and efficacious method whereby the durability of flame retardant properties of textile materials treated with phosphorus-containing agents affixed to the material in an amount of from about 0.5 to about 5, percent of phosphorus based on the weight of the material is improved by the aftertreatment of the textile material with a salt of a heavy metal or a transition metal, i.e., a metal which is in Group I-B, IV-A, IV-B, V-A, V-B, VII-B and VIII of the Periodic Table of Elements. Titanyl sulfate has been found to be a particularly efficacious metal salt for use in that aftertreatment process.
The treatment disclosed in that aforesaid application Ser. No. 307,796, serves to increase the flameproofing effect of the flameproofing agent and protect it against ion exchange or other effects of exposure to hard water, thereby promoting flame resistance and increasing its retention over a large number of launderings.
While the treatment disclosed in the aforesaid application represents a substantial improvement in the art, the search has continued for improved methods for imparting durable flame retardance to cellulosic fiber-containing textile materials. It has been found, for example, that treatments applied to a textile material to obtain acceptable flame retardance properties may adversely affect other properties of the textile material. The textile material may shrink a substantial amount (e.g., about 10 or 20 percent or more) during treatment with a phosphorus-containing flame retardant material as compared with an untreated textile material. Other properties of the textile material may also be adversely affected.